1. Field of the Invention
The present invention generally relates to a steering system for vehicles. More particularly, the invention relates to an electric power steering system for vehicles.
2. Description of Relevant Art
In view of the problems of hydraulic type power steering system for vehicles because of a complicated structure, in recent years a variety of electric type power steering systems for vehicles have been proposed.
Those electric type power steering systems were all of a type including an input shaft such as a steering shaft connected to a steering wheel, an output shaft interconnected with the input shaft and, through a steering gear mechanism of a desired type, with a tie rod of a road wheel to be steered. Also provided were an electric motor for supplying the output shaft with auxiliary torque, a torque detection mechanism for detecting the magnitude as well as the direction of steering torque acting on the input shaft, and a driving control circuit for feeding the electric motor with an armature current of such a magnitude and in such a direction as necessary in accordance with a detection signal from the torque detection mechanism.
In the abpve-mentioned power steering systems, when the steering wheel was operated, the output shaft was supplied with an adequate auxiliary torque, thereby facilitating the steering operation.
As an example of such an electric power steering system for vehicles is one disclosed by Japanese Patent Application Lay-Open Print No. 59-70257 (laid open on Apr. 20, 1984), and another is disclosed by Japanese Patent Application Lay-Open Print No. 59-227560 (laid open on Dec. 20, 1984).
In the electric power steering system according to the first Lay-Open Print application, the steering gear mechanism was of a rack and pinion type and the torque detection mechanism was constituted with a strain gauge sensor disposed on the input shaft. This power steering system had produced, based on an output signal from the strain gauge sensor, a torque direction signal and a torque magnitude signal representing the direction in which the steering torque was acting on the input shaft and the magnitude of this steering torque, respectively, while sending, in accordance with these torque representative signals, the armature current of the required magnitude and in the required direction to the electric motor to thereby supply the output shaft with adequate auxiliary torque.
In the electric power steering system according to the second Lay-Open Print application, the steering gear mechanism was of a worm shaft and ball nut type. The input shaft was interconnected through a torsion bar with the output shaft, while the torque detection mechanism was constituted with a sliding resistance member secured to the input shaft and a contact piece secured to the output shaft. The contact piece were put in slidable contact with the sliding resistance member. Also this power steering system produced, based on an output signal from the sliding-resistance type torque detection mechanism, a torque direction signal and a torque magnitude signal, while sending, in accordance with these torque representative signals, the armature current to the electric motor, similar to the steering system according to the former Lay-Open Print.
In both examples according to the aforesaid Japanese Patent Application Lay-Open Prints, the torque direction signal and the torque magnitude signal produced were based on the output signal from the torque detection mechanism, that is, a pair of torque representative signals were generated in accordance with a detection signal from a single signal source.
Typically in a vehicle having a front steered ground wheel, when the direction of travel is changed while the vehicle is traveling straight, the front wheel is turned in either direction from a neutral position thereof by operating a steering wheel, thereby turning.
In such a turn, based on the wheel alignment of the front wheel as well as self-aligning torque due to deformations of a tire, righting moment acts on the front wheel, with a tendency to return the front wheel to the neutral position.
Where the steering system of such a vehicle is of an ordinary type having no auxiliary torque supplied thereto, the strength of steering forces applied to the steering wheel by the driver's hands is gradually reduced to zero while the vehicle is turning causing the front wheel to move with the righting moment to gradually returning to the neutral position. When the front wheel is returned to the neutral position, the vehicle again travels in a forward straight direction.
Such a state of transition of travelling is frequently experienced when turning corners with a vehicle equipped with an ordinary type steering system.
On the other hand, if the vehicle is equipped with either of the electric power steering systems according to the aforesaid Japanese Patent Application Lay-Open Prints, the possibility of experiencing a problem as described below is found when returning from turning state to a straight travel.
A description will now be made of an exemplary case in which a vehicle returns from a rightwardly turning state to traveling straight.
In the rightwardly turning state of the vehicle, the steering wheel has steering forces applied thereto through the driver's hands with a tendency to produce clockwise rotation thereof. Under such a condition, as the strength of the steering forces is substantially reduced to zero in a relatively short time by gradually releasing or lightly holding the steering wheel, the front wheel tends to return to the neutral position within a relatively short time, by the effect of a righting moment acting on the rightwardly turned front wheel, which turns the wheel to the left or in the counterclockwise direction when viewed from above, that is, from the side of the driver. Concurrently, however, the output shaft of the power steering system, to which the righting moment is transmitted in the form of counterclockwise torque, is caused to quickly rotate counterclockwise. Such counterclockwise rotation of the output shaft corresponds to a clockwise rotation of the input shaft relative to the output shaft, so that the torque detection mechanism detects false steering torque acting clockwise about the input shaft. Accordingly, clockwise auxiliary torque of a certain magnitude is supplied to the output shaft from the electric motor. Thus, the front wheel slightly is turned to the right, resulting in a delay of the returning action of the front wheel as well as of the steering wheel to the neutral position.
Such a problem resulted from the constitution of the electric power steering systems according to the aforesaid Japanese Patent Application Lay-Open Prints in which both the torque direction signal and the torque magnitude signal were produced in accordance with the detection signal from one torque detection mechanism.
The present invention effectively solves the aforementioned problem of conventional electric power steering systems for vehicles.